1. Field of the invention
The present invention relates to an electrode conductive material and more particularly to the conductive material used in the positive electrode of an alkaline electrolyte secondary storage cell.
2. Description of the prior art
A non-sintered nickel electrode (also referred to as a pasted or plasticized electrode) includes a support serving as a current collector and coated with a paste containing the active material and a binder, to which a conductive material is usually added. During fabrication of the electrode a volatile solvent is added to the paste to modify its viscosity in order to facilitate shaping. When the paste has been deposited on or in the support, the combination is compressed and dried to obtain an electrode with the required density and of the required thickness.
The active material in an alkaline electrolyte storage cell positive electrode is usually a nickel-based hydroxide. Nickel hydroxide is a poor conductor and requires the addition to the electrode of a material enabling good electrical percolation. The paste therefore usually contains a conductive material, which can in particular be a cobalt compound such as metallic cobalt Co, cobalt hydroxide Co(OH)2 and/or a cobalt oxide such as cobalt monoxide CoO. For example, it has been proposed to use as the conductive material a non-stoichiometric cobalt oxide with the formula CoxO where 0.93xe2x89xa6xc3x97xe2x89xa60.97, a cobalt oxide with the formula CoOx where 0 less than xc3x97 less than 1 or a cobalt monoxide CoO in which the surface of the particles is covered with hydroxyl groups or a layer of cobalt oxide with a valency at least equal to 2, such as Co2O3 or Co3O4.
When an alkaline storage cell is charged for the first time, the above substances are oxidized to form cobalt oxyhydride CoOOH in which the degree of oxidation of the cobalt is +3 or greater. Cobalt oxyhydride is stable in the normal range of operating conditions of the nickel positive electrode and insoluble in the alkaline electrolyte. It assures the electrical percolation of the electrode.
When stored in a completely discharged state, the voltage of an alkaline storage cell having a non-sintered nickel positive electrode decreases with time. If the storage time exceeds a few months, the voltage tends towards 0 V. Under these conditions the cobalt oxyhydride is reduced slowly. The degree of oxidation of the cobalt first falls to +2.66, in Co3O4, and then +2, in Co(OH)2. Cobalt oxyhydride Co(OH)2 is highly soluble in the electrolyte. Consequently, after a storage period of several months, a loss of conductivity is observed due to partial dissolution of the percolating network of the non-sintered electrode. This causes an irreversible loss of capacity, which can exceed 15%. This irreversible loss of capacity occurs whichever cobalt compound known in the art is introduced into the paste.
The object of the present invention is to propose a conductive material for a non-sintered nickel electrode such that the irreversible loss of capacity of the electrode in storage is greatly reduced compared to the electrodes known in the art, in combination with a high yield.
The present invention provides an electrode conductive material including a non-stoichiometric oxidized cobalt compound whose electrical conductivity is greater than 5xc3x9710xe2x88x923 S/cm and which has a direct spinel structure and formula Co2+xCo3+yO4, with 0.73xe2x89xa6xxe2x89xa60.80 and 1.87 xe2x89xa6yxe2x89xa61.95.
The above substance is isomorphic with spinel (magnesium aluminate MgAl2O4). Its structure is based on a cubic unit cell with eight face-centered oxygen lattice units. In this instance the divalent Co2+ ions are in the tetrahedral sites of the cubic lattice, the trivalent Co3+ ions are in the octahedral sites of the lattice, and the oxygen ions are at the apices of the tetrahedrons and the octahedrons. This structure is referred to as the xe2x80x9cnormalxe2x80x9d or xe2x80x9cdirectxe2x80x9d structure, as compared with the inverse structure in which the divalent and trivalent ions are disposed differently. In a structure of this type corresponding to the general formula AB2O4, there are usually twice as many trivalent ions as divalent ions. One important feature of the conductive material according to the present invention is that the cobalt oxide is non-stoichiometric, i.e. there is a cobalt deficit.
The method of fabricating an electrode conductive material according to the present invention, as previously described, includes the following steps: a basic solution made up of a mixture of potassium hydroxide KOH and sodium hydroxide NaOH is introduced into a reactor and stirred mechanically, a constant flow of oxygen is established in said solution, an initial powder containing cobalt hydroxide is introduced into said solution and left in contact with said solution, a final powder is separated from said solution, and said final powder is washed and dried.
Said solution is preferably a mixture of 6N potassium hydroxide KOH and 4N sodium hydroxide NaOH. The purpose of mechanical stirring is to take the powder up into suspension in the solution. Stirring can be obtained by means of a paddle-wheel, for example. For the reaction to be homogeneous, the flow of oxygen preferably enters the reactor from the bottom to improve the distribution of oxygen in the solution.
For example, the temperature of the solution can be from 80xc2x0 C. to 120xc2x0 C. and the time for which the powder is in contact with the solution can be from 5 hours to 48 hours.
In a first variant, the initial powder is cobalt hydroxide.
In another variant, the initial powder consists of particles based on nickel hydroxide covered with a layer of cobalt hydroxide.
When the reaction has taken place, the powder finally obtained is separated from the solution. The final powder is then washed, preferably with water, and then dried at moderate temperature, for example in a vacuum.
The present invention also provides an alkaline electrolyte secondary storage cell positive electrode including a current collector and a layer containing a binder, particles of an electrochemically active material and a conductive material including a non-stoichiometric oxidized cobalt compound whose electrical conductivity is greater than 5xc3x9710+3 S/cm and which has a direct spinel structure and formula Co2+xCo3+y O4, with 0.73xe2x89xa6x less than 0.80 and 1.87xe2x89xa6yxe2x89xa61.95, the proportion by weight of said conductive material in said layer being from 4% to 10% of said active material.
In a first variant, the conductive material consists of particles. In the layer containing a binder, particles of the powder conductive material are mixed with particles of said active material and the binder. Preferably the average diameter of the particles of the conductive material is less than 2 xcexcm.
In a second variant, the conductive material consists of a coating covering the particles of the active material. The active material particles are therefore coated with a layer of the conductive material.
In a third variant, the conductive material consists of particles mixed with the particles of the active material and of a coating covering the particles of the active material.
The current collector can be a two-dimensional conductive support, such as solid or perforated strip, expanded metal, grid or woven material, or a porous three-dimensional conductive support, such as felt or foam. The support can be based on metal or carbon.
The binder includes at least one substance chosen from carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), a copolymer of styrene, ethylene, butylene and styrene (SEBS), a terpolymer of styrene, butadiene and vinylpyridine (SBVR), a copolymer of styrene and butadiene (SBR), possibly carboxylated, a copolymer of acrylonitrile and butadiene (NBR), polytetrafluoroethylene (PTFE), a fluorinated copolymer of ethylene and propylene (FEP), polyhexafluoropropylene (PPHF), polyvinylidene fluoride (PVDF), and ethylvinyl alcohol (EVA).
In a first variant the binder is a mixture of a crystalline polymer and an elastomer.
The crystalline polymer can be chosen from a fluorinated polymer, such as polytetrafluoroethylene (PTFE), a fluorinated copolymer of ethylene and propylene (FEP), polyhexafluoropropylene (PPHF), and polyvinylidene fluoride (PVDF).
The elastomer can be chosen from a copolymer of styrene, ethylene, butylene and styrene (SEBS), a terpolymer of styrene, butadiene and vinylpyridine (SBVR), a copolymer of styrene and butadiene (SBR) and a copolymer of acrylonitrile and butadiene (NBR).
In a second variant, the binder includes a first substance in the form of a fluorinated polymer and at least one second substance chosen from a cellulose compound, a fluorinated compound, an elastomer and ethylvinyl alcohol (EVA).
The fluorinated polymer can be chosen from polytetrafluoroethylene (PTFE), a fluorinated copolymer of ethylene and propylene (FEP), polyhexafluoropropylene (PPHF) and polyvinylidene fluoride (PVDF).
The cellulose compound can be chosen from carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), hydroxyethylcellulose (HEC) and hydroxypropylcellulose (HPC).
The fluorinated compound can be chosen from polytetrafluoroethylene (PTFE), a fluorinated copolymer of ethylene and propylene (FEP), polyhexafluoropropylene (PPHF), and polyvinylidene fluoride (PVDF).
The elastomer can be chosen from a copolymer of styrene, ethylene, butylene and styrene (SEBS), a terpolymer of styrene, butadiene and vinylpyridine (SBVR), a copolymer of styrene and butadiene (SBR), and a copolymer of acrylonitrile and butadiene (NBR).
In a third variant the binder is a mixture of ethylvinyl alcohol (EVA) and an elastomer.
The elastomer can be chosen from a copolymer of styrene, ethylene, butylene and styrene (SEBS), a terpolymer of styrene, butadiene and vinylpyridine (SBVR), a copolymer of styrene and butadiene (SBR), and a copolymer of acrylonitrile and butadiene (NBR).
The electrochemically active material is a nickel-based hydroxide. The expression xe2x80x9cnickel-based hydroxidexe2x80x9d means a hydroxide of nickel or a hydroxide containing mainly nickel, in particular a hydroxide of nickel containing at least one syncrystallized hydroxide of an element chosen from zinc (Zn), cadmium (Cd), magnesium (Mg), aluminum (Al) and cobalt (Co), and at least one syncrystallized hydroxide of an element chosen from cobalt (Co), manganese (Mn), aluminum (Al), yttrium (Y), calcium (Ca), strontium (Sr), zirconium (Zr), and copper (Cu). A syncrystallized hydroxide contained in the nickel hydroxide is a hydroxide forming a solid solution with the nickel hydroxide, i.e. one occupying in continuously variable proportions the atomic sites defined by the crystal lattice of the nickel hydroxide.
The paste can further contain at least one other substance chosen from zinc compounds such as ZnO or Zn(OH)2, yttrium compounds such as Y2O3 or Y(OH)3, and calcium compounds such as CaO, Ca(OH)2, or CaF2. The substance is usually added in powder form.
To facilitate the manufacture of the electrode, the paste can further contain a thickener, in particular a cellulose compound chosen from the sodium salt of carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), hydroxypropylcellulose (HPC) and hydroxyethylcellulose (HEC).
A positive electrode in accordance with the invention can be used in any alkaline electrolyte storage cell, for example nickel-metal hydride, nickel-cadmium, nickel-iron, nickel-zinc and nickel-hydrogen storage cells.